guide to gc column selection and optimizing separations brochure

ID

WHICH COLUMN DO I NEED?

STATIONARYPHASE

ID FILMTHICKNESSID

INNERDIAMETERID

LENGTH

ID

Guide to GC Column Selection and Optimizing

Separations

• Learnhowtochoosetherightcolumnthefirsttime.• Optimizeseparationsforthebestbalanceofresolutionandspeed.• Troubleshootquicklyandeffectivelybasedonchromatographicsymptoms.

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You can improve lab productivity by assuring that speed and resolution are optimized. One of the best ways to do this is to use the resolution equation (Figure 1) as the key to controlling your separations. This fundamental equation helps you choose the best column stationary phase, length, inner diameter (ID), and film thickness for your specific applications. Once you understand the basics of how resolution is related to column characteristics, optimizing your analysis for both separation and speed becomes easier. This GC column selection guide discusses the basics of separation and teaches you how to choose the right GC column!

Resolution is the goal of every chromatographer, but how much resolution is enough? Practically speaking, we need enough reten-tion to get sharp symmetrical peaks that are baseline resolved from each other, but not too much retention, where retention times are too long and peaks start to broaden. To achieve this goal, we must consider the column and non-column factors that affect our “perfect separation”. Only then can we work towards selecting the right column and optimizing GC separations and analysis speed. Now, let’s consider separation factor (α), retention factor (k), and efficiency (N) in turn and how they can help you select the right column and optimize your separation.

Lookforapplication-specificstationaryphasesfirst;thesecolumnsareoptimizedforspecificanalysesandwillprovidethebestresolutionintheshortesttime(TableIII).

Ifanapplication-specificcolumnisnotavailableandyouneedtomeasurelowconcentrationsorareusingamassspectrometer(MS),thenchooseanRxi®column.Rxi®technologyunitesoutstandinginertness,lowbleed,andhighreproducibility,resultinginhigh-performanceGCcolumnsthatareidealfortraceanalysisandMSwork(TableII).

Forothermethods,chooseageneral-purposeRtx®column(TableII).

Shortcut to Column Selection

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Use Separation Factor (α) to Choose the Best Stationary PhaseChoosing the right stationary phase is the first step toward optimizing your GC separation. It is the most important decision you will make because separation factor (α) has the greatest impact on resolu-tion, and it is strongly affected by stationary phase polarity and selectivity.

Stationary phase polarity is determined by the type and amount of functional groups in the stationary phase. When choosing a column, consider the polarity of both the stationary phase and your target analytes. If the stationary phase and analyte polarities are similar, then the attractive forces are strong and more retention will result. Greater retention often results in increased resolu-tion. Stationary phase polarity strongly influences column selectivity and separation factor, making it a useful consideration when selecting a column.

Stationary phase selectivity is defined by IUPAC as the extent to which other substances interfere with the determination of a given substance. Selectivity is directly related to stationary phase composition and how it interacts with target compounds through in-termolecular forces (e.g., hydrogen bonding, dispersion, dipole-dipole interactions, and shape selectivity). As methyl groups in the stationary phase are replaced by different functionalities, such as phenyl or cyanopropyl pendant groups, compounds that are more soluble with those functional groups (e.g., aromatics or polar compounds, respectively) will interact more and be retained longer,

Figure 1:Theresolutionequationandfactorsthataffectit.

=R N14

kk+1

α-1X X

• Length• Inner diameter• Carrier gas type and linear velocity

• Inner diameter• Film thickness• Temperature

N = L/H = E�ective theoretical plate numberL = Column lengthH = HETP = Height equivalent to a theoretical plate

k = Retention factorα = Separation factorBaseline resolution (R = 1.5) is the goal.

• Stationary phase composition• Temperature

A measure of E�ciency.This term is a�ected by:

A measure of Retention.This term is a�ected by:

A measure of Peak Separation.This term is a�ected by:

ResolutionEquation—TheKeytoColumnSelectionandOptimizingGCSeparations

=R N14

kk+1

1X X

=R N14

kk+1

α-1X X

=R N14

kk+1

α-1X X

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Table I:Kovat’sretentionindicesforGCphasescanbeusedtoapproximateselectivity.

Inmanycases,differentGCoventemperatureprogramscanchangetheelutionorderofsampleanalytesonthesamecolumn.ReconfirmelutionordersifchangingGCoventemperatureprograms.

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Anyhomologousseriesofcompounds,thatis,analytesfromthesamechemicalclass(e.g.,allalcohols,allketones,orallaldehydes,etc.)willeluteinboilingpointorderonanystationaryphase.However,whendifferentcompoundclassesaremixedtogetherinonesample,intermolecularforcesbetweentheanalytesandthestationaryphasearethedominantseparationmechanism,notboilingpoint.

often leading to better resolution and increased selectivity. In another example of the effect of stationary phase-analyte interactions, an Rtx®-200 stationary phase is highly selective for analytes containing lone pair electrons, such as halogen, nitrogen, or carbonyl groups, due to interactions with the fluorine pen-dant group in this phase. Selectivity can be approximated using existing applications or retention indices (Table I), making these useful tools for comparing phases and deciding which is most appropriate for a specific analysis.

Due to their influence on separation factor, polarity and selectivity are primary considerations when selecting a column. However, temperature limits must also be considered. In general, highly polar sta-tionary phases have lower maximum operating temperatures, so choosing a column with the appropriate maximum operating temperature as well as optimal polarity and selectivity for the type of compounds being analyzed is crucial. Use Table II and Figure 2 determine which general-purpose column is most ap-propriate based on the selectivity, polarity, and the temperature requirements of your analysis. See Table III for a list of specialty stationary phases designed for specific applications.

tech tip

Stationary Phase Benzene Butanol Pentanone Nitropropane100% Dimethyl polysiloxane 651 651 667 7055% Diphenyl/95% dimethyl polysiloxane 667 667 689 74320% Diphenyl/80% dimethyl polysiloxane 711 704 740 8206% Cyanopropylphenyl/94% dimethyl polysiloxane 689 729 739 81635% Diphenyl/65% dimethyl polysiloxane 746 733 773 867Trifluoropropylmethyl polysiloxane 738 758 884 980Phenyl methyl polysiloxane 778 769 813 92114% Cyanopropylphenyl/86% dimethyl polysiloxane 721 778 784 88165% Diphenyl/35% dimethyl polysiloxane 794 779 825 93850% Cyanopropylmethyl/50% phenylmethyl polysiloxane 847 937 958 958Polyethylene glycol 963 1,158 998 1,230

Small ID Large ID Nonpolar

100% Dimethyl

Rxi®-1msRxi®-1HT

Rtx®-1

PolyethyleneGlycol

Rtx®-WaxStabilwax®

90% + Bis-Cyano

Rtx®-2330

Proprietary(Silarylene) Rxi®-624Sil MS


35% Diphenyl Rtx®-35

6% Cyano Rtx®-624

Rtx®-1301


5% Diphenyl Rxi®-5msRxi®-5HT

Rtx®-5


50% Diphenyl Rxi®-17

Proprietary

Rxi®-XLB

Proprietary

Rtx®-440

Tri�uoropropyl

Rtx®-200Rtx®-200MS

14% Cyano

Rtx®-1701

50% Cyano

Rtx®-225

65% Diphenyl

Rtx®-65

20% Diphenyl

Rtx®-20

50% Phenyl

Rtx®-50

Short Length Long Length Thin Film Thick Film

Characteristics• Shorter retention times• Lower bleed• Higher maximum temperatures• Lower sample loading capacity• High resolution for high molecular weight compounds

Characteristics• Longer retention times• Higher bleed• Lower maximum temperatures• Higher sample loading capacity• High resolution for volatiles and low molecular weight compounds

ApplicationsMedium and high molecular weight compounds Applications

• Volatile, low molecular weight compounds• High concentration samples (e.g., purity testing)

Characteristics• Good e�ciency• Short analysis times

Characteristics• Better e�ciency• Moderate analysis times

ApplicationsSamples with fewcompounds

ApplicationsMore complex samples

Characteristics• Best e�ciency• Longer analysis times

ApplicationsVery complex samples

Characteristics• Highest e�ciency• Shorter analysis time• Lower sample loading capacity

Characteristics• High e�ciency• Good performance for analysis time and sample loading capacity

Applications• Highly complex samples • Fast GC• GC-MS• Split injection

Applications• Complex samples• Wide concentration range• Split , splitless, direct, headspace, and on-column injection

Characteristics• Good e�ciency• Longer analysis time• Higher sample loading capacity• May require higher �ow rates than MS detectors can tolerate

Applications• Packed column replacement• Purity analysis• Split, splitless, direct, headspace, and on-column injection

As �lm thickness increases, retention, sample loading capacity,and column bleed increase; whereas, maximum temperature decreases.

In general, maximum operating temperature decreases as polarity increases.Note that silarylene columns typically di�er in selectivity and have higher temperature limits than their conventional counterparts.

As inner diameter increases, e�ciency decreases, sample loading capacity increases, optimal �ow rate increases, and analysis time increases.

Polar

Longer Columns Can Increase Resolution...Doubling the column length only increases resolution by approximately 40% because the column length is under the square root function in the e�ciency term of the resolution equation.

But, Longer Columns Increase Cost and Analysis Time On longer columns, analysis time is increased by as much as a factor of two.Longer columns are also more expensive.

Figure 2:Polarityscaleofcommonstationaryphases.

STATIONARY PHASE

ID

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Table II: RelativepolarityandthermalstabilityareimportantconsiderationswhenselectingaGCstationaryphase.

Restek Phase Composition (USP Nomenclature)Restek’s Max Temps* Agilent Phenomenex

Rxi-1HT Rxi-1ms, Rtx-1 100% Dimethyl polysiloxane (G1, G2, G38)

400 °C350 °C

HP-1/HP-1ms, DB-1/DB-1ms, VF-1ms, CP Sil 5 CB, Ultra 1, DB-1ht, HP-1ms UI, DB-1ms UI

ZB-1, ZB-1MS, ZB-1HT Inferno

Rxi-5HT Rxi-5ms, Rtx-5 5% Diphenyl/95% dimethyl polysiloxane (G27, G36)

400 °C350 °C

HP-5/HP-5ms, DB-5, Ultra 2, DB-5ht, VF-5ht, CP-Sil 8 CB

ZB-5, ZB-5HT Inferno, ZB-5ms

Rxi-5Sil MS5% (1,4-bis(dimethylsiloxy) phenylene/95% dimethyl polysiloxane 350 °C

DB-5ms UI, DB-5ms, VF-5ms ZB-5msi

Rxi-XLB Proprietary Phase 360 °C DB-XLB, VF-Xms MR1, ZB-XLBRtx-20 20% Diphenyl/80% dimethyl polysiloxane (G28, G32) 320 °C — —Rtx-35 35% Diphenyl/65% dimethyl polysiloxane (G42) 320 °C HP-35, DB-35 ZB-35

Rxi-35Sil MS Proprietary Phase 360 °CDB-35ms, DB-35ms UI, VF-35ms MR2

Rtx-50 Phenyl methyl polysiloxane (G3) 320 °C — —

Rxi-17 50% Diphenyl/50% dimethyl polysiloxane 320 °CDB-17ms, VF-17ms, CP Sil 24 CB ZB-50

Rxi-17Sil MS Proprietary Phase 360 °CDB-17ms, VF-17ms, CP Sil 24 CB ZB-50

Rtx-65 65% Diphenyl/35% dimethyl polysiloxane (G17) 300 °C — —

Rxi-624Sil MS Proprietary Phase 320 °CDB-624 UI, VF-624ms, CP-Select 624 CB ZB-624

Rtx-1301,Rtx-624 6% Cyanopropylphenyl/94% dimethyl polysiloxane (G43)

280 °C240 °C

DB-1301, DB-624, CP-1301, VF-1301ms, VF-624ms ZB-624

Rtx-1701 14% Cyanopropylphenyl/86% dimethyl polysiloxane (G46) 280 °C

DB-1701, VF-1701ms, CP Sil 19 CB, VF-1701 Pesticides, DB-1701R ZB-1701, ZB-1701P

Rtx-200, Rtx-200MS Trifluoropropyl methyl polysiloxane (G6) 340 °C DB-200, VF-200ms, DB-210 —

Rtx-22550% Cyanopropyl methyl/50% phenylmethyl polysiloxane (G7, G19) 240 °C DB-225ms, CP Sil 43 CB —

Rtx-440 Proprietary Phase 340 °C

Rtx-233090% Biscyanopropyl/10% cyanopropylphenyl polysiloxane (G48) 275 °C VF-23ms —

Rt-2560 Biscyanopropyl polysiloxane 250 °C HP-88, CP Sil 88 —Rtx-Wax Polyethylene glycol (G14, G15, G16, G20, G39) 250 °C DB-Wax, Wax 52 CB ZB-WAXStabilwax Polyethylene glycol (G14, G15, G16, G20, G39) 260 °C HP-INNOWax, VF-WaxMS ZB-WAXPlus

Table III:Application-specificphasesdesignedforparticularanalyses.

* Maximum operating temperatures may vary with column film thickness.

Restek Applications Agilent Supelco Macherey-Nagel SGE PhenomenexRtx-Volatile Amine Volatile amines CP-VolAmine — — — —Rtx-5Amine Amines CP-Sil 8 CB — OPTIMA 5 Amine — —Rtx-35Amine Amines — — —Stabilwax-DB Amines CAM, CP WAX 51 Carbowax Amine FS-CW 20 M-AM — —

Stabilwax-DA Free fatty acids

HP-FFAP, DB-FFAP, VF-DA, CP WAX58 CB, CP-FFAP CB Nukol

PERMABOND FFAP,OPTIMA FFAP, OPTIMA FFAP Plus BP-21 ZB-FFAP

Chiral ColumnsRt-βDEXm, Rt-βDEXsm,Rt-βDEXse, Rt-βDEXsp,Rt-βDEXsa, Rt-βDEXcst,Rt-γDEXsa Chiral compounds — — — — —Foods, Flavors, & FragrancesRt-2560 cis/trans FAMEs HP-88 SPB-2560 — — —FAMEWAX Marine oils Select FAME Omegawax — — —Rtx-65 TG Triglycerides — — — — —

Rxi-PAHPolycyclic aromatic hydrocarbons (PAHs) Agilent Select PAH — — — —

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Table III:Application-specificphasesdesignedforparticularanalyses(cont.)

Restek Applications Agilent Supelco Macherey-Nagel SGE PhenomenexPetroleum & Petrochemical

Rt-Alumina BOND/CFCChlorinated fluorocarbons (CFCs) — — —

Rtx-DHADetailed hydrocarbon analysis

HP-PONA, DB-Petro, CP Sil PONA CB Petrocol DH — BP1PONA —

Rtx-2887Hydrocarbons (ASTM D2887) DB-2887

Petrocol 2887, Petrocol EX2887 — — —

MXT-2887Hydrocarbons (ASTM D2887) DB-2887

Petrocol 2887, Petrocol EX2887 — — —

D3606Ethanol (ASTM D3606) — — —

Rt-TCEPAromatics and oxygenates in gasoline CP-TCEP TCEP — — —

MXT-1HT SimDist Simulated distillationDB-HT-SimDis,CP-SimDist, CP-SimDist Ultimetal — — BPX1 ZB-1XT SimDist

MXT-1 SimDist Simulated distillationDB-HT-SimDis,CP-SimDist, CP-SimDist Ultimetal — — — —

MXT-500 SimDist Simulated distillation — — —Rtx-Biodiesel TG, MXT-Biodiesel TG Triglycerides in biodiesel Biodiesel, Select Biodiesel — OPTIMA Biodiesel — ZB-BioethanolClinical/ForensicRtx-BAC Plus 1 Blood alcohol testing DB-ALC1 — — — ZB-BAC1Rtx-BAC Plus 2 Blood alcohol testing DB-ALC2 — — — ZB-BAC2Pharmaceutical

Rtx-G27 w/IntegraGuardOrganic volatile impurities (USP <467>) — — — — —

Rtx-G43 w/IntegraGuardOrganic volatile impurities (USP <467>) — — — — —

Rxi-624Sil MSOrganic volatile impurities (USP <467>)

DB-624,VF-624ms, CP-Select 624 CB — OPTIMA 624 LB BP624 ZB-624

Rtx-5 (G27)Organic volatile impurities (USP <467>) HP-5, DB-5,CP Sil 8 CB SPB-5 OPTIMA 5 BP5 ZB-5

Stabilwax (G16)Organic volatile impurities (USP <467>)

HP-INNOWax,CP Wax 52 CB,VF-WAX MS Supelcowax-10 OPTIMA WAXplus — ZB-WAXplus

Environmental

Rxi-5Sil MS SemivolatilesDB-5ms,DB-5msUI, VF-5ms,CP-Sil 8 CB SLB-5ms OPTIMA 5MS Accent BPX5 ZB-5msi

Rtx-VMSVolatiles (EPA Methods 8260, 624, 524) — — —

Rxi-624Sil MSVolatiles (EPA Methods 624)

DB-624,VF-624ms, CP-Select 624 CB — OPTIMA 624 LB BP624 ZB-624

Rtx-502.2Volatiles (EPA Methods 8010, 8020, 502.2, 601, 602) DB-502.2 VOCOL — — —

Rtx-VolatilesVolatiles (EPA Methods 8010, 8020, 502.2, 601, 602) — VOCOL — — —

Rtx-VRXVolatiles (EPA Methods 8010, 8020, 502.2, 601, 602) DB-VRX — — — —

Rtx-CLPesticides Organochlorine pesticides — — —Rtx-CLPesticides2 Organochlorine pesticides — — —Rtx-1614 Brominated flame retardants — — —

Rtx-PCBPolychlorinated biphenyl (PCB) congeners — — —

Rxi-XLBPolychlorinated biphenyl (PCB) congeners DB-XLB,VF-XMS — — — MR1, ZB-XLB

Rtx-OPPesticidesOrganophosphorus pesticides — — —

Rtx-OPPesticides2Organophosphorus pesticides — — —

Rtx-Dioxin2 Dioxins and furans — — —

Rxi-17Sil MSPolycyclic aromatic hydrocarbons (PAHs)

DB-17ms,VF-17ms, CP-Sil 24 CB — OPTIMA 17 MS BPX50 ZB-50

Rtx-Mineral Oil DIN EN ISO 9377-2 Select Mineral Oil — — — —

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Select Column Film Thickness and Column ID Based on Retention FactorOnce you have chosen the stationary phase, you need to determine which column film thickness and inner diameter combination will give the retention factor (k) needed for optimal resolution and speed. Retention factor is sometimes referred to as capacity factor, which should not be confused with sample loading capacity.

The retention factor (k) of a column is based on the time an analyte spends in the stationary phase rela-tive to the time it spends in the carrier gas. As a general rule, the thicker the film and the smaller the inner diameter, the more an analyte will be retained. Note that as temperature increases k decreases, so at higher temperatures analytes stay in the carrier gas longer and are less retained.

In practice, if the value of k is too large, the peak will broaden, which can reduce resolution by causing peaks to overlap or coelute. Narrow, symmetrical peaks are important to maximizing resolution, so the goal is to select a column with a sufficient retention factor, such that resolution occurs and peak shape does not suffer. Once the proper stationary phase is selected, column film thickness, column inner diam-eter, and elution temperature should be optimized to produce an acceptable retention factor.

Film Thickness

Film thickness (μm) has a direct effect on both the retention of each sample component and the max-imum operating temperature of the column. When analyzing extremely volatile compounds, a thick film column should be used to increase retention; more separation is achieved because the compounds spend more time in the stationary phase. If analyzing high molecular weight compounds, a thinner film column should be used, as this reduces the length of time that the analytes stay in the column and minimizes phase bleed at higher elution temperatures. Use Figure 3 to select the best film thickness for your application. Note that as a general rule, the thicker the film, the lower the maximum temperature; exceeding the maximum temperature can result in column bleed and should be avoided.

Thesampleloadingcapacityofthecolumnalsomustbeconsidered;ifthemassofthetargetanalyteexceedsthesampleloadingcapacityofthecolumn,lossofresolution,poorreproducibility,andfrontingpeakswillresult.AlargerIDcolumnwiththickerfilmisrecommendedforhigherconcentrationsamples,suchaspurityanalysis,tominimizesampleoverload.

Remember,whenchangingeitherfilmthicknessand/orthetemperatureprogram,youmustreconfirmpeakidentificationsaselutionorderchangescanoccur.

tech tip

tech tip


100% Dimethyl

Rxi®-1msRxi®-1HT

Rtx®-1

PolyethyleneGlycol


90% + Bis-Cyano

Rtx®-2330




6% Cyano Rtx®-624

Rtx®-1301



Rtx®-5



Proprietary

Rxi®-XLB

Proprietary

Rtx®-440

Tri�uoropropyl


14% Cyano

Rtx®-1701

50% Cyano

Rtx®-225

65% Diphenyl

Rtx®-65

20% Diphenyl

Rtx®-20

50% Phenyl

Rtx®-50





















Polar



=R N14

kk+1

1X X

=R N14

kk+1

α-1X X

=R N14

kk+1

α-1X X

Figure 3:Characteristicsandrecommendedapplicationsbasedonfilmthickness.

FILM THICKNESSID

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Inner Diameter (ID)

Column ID does not have as great an effect on retention factor as film thickness does. However, when se-lecting column ID with retention factor (k) in mind, a general rule of thumb applies; smaller ID columns produce higher retention factors compared to larger ID columns. This is due to less available mobile phase (carrier gas) volume in the column. Because smaller ID columns produce higher k values, they are more suited towards complex sample analysis where a range of low to high molecular weight compounds may exist in the sample (Figure 4). Keep in mind that both ID and film thickness should be optimized together to produce the best resolution and peak shape.

WhenchoosingcolumnID,theinjectiontechniqueisalsoim-portantbecausethecolumnIDmayneedtobeselectedbasedonwhetherasplit,splitless,direct,coolon-columninjec-tion,orothersampletransfermethodisbeingused.Forex-ample,0.53mmIDcolumnsareidealforcoolon-columninjec-tionssincethesyringeneedle(26gauge)willfitintothelargecolumnID.Inaddition,thedetectoranditsoptimalflowratemustbeconsidered.SomeMSdetectorscanonlyoperateundercolumnflowratesofupto1.5mL/min;therefore,a0.53mmIDcolumn,whichrequireshigherflowsforproperchroma-tography,isnotanoptionforMSwork.

tech tip

Phase Ratio (β)

The relationship between column inner diameter and stationary phase film thickness is expressed as phase ratio (β). If a good separation has been achieved on a larger diameter column and a faster analysis is desired, this can often be accomplished by reducing the inner diameter of the column without sacri-ficing, and sometimes even improving, separation efficiency. To maintain a similar compound elution pattern when narrowing column inner diameter, film thickness must also be changed. By choosing a column with a similar phase ratio, it will be easier to translate your application to the new column. Phase ratios for common column dimensions and the equation for β are given in Table IV. As shown here, an analyst wanting to decrease analysis time could switch from a 0.32 mm x 0.50 µm column (β = 160) to a 0.25 mm x 0.25 μm column (β = 250) and obtain a very similar separation upon proper method transla-tion. Importantly, column inner diameter and stationary phase film thickness show a combined effect when it comes to sample loading capacity, which is decreased as column inner diameter and film thick-ness are reduced. It may be necessary to inject a lower sample amount in this case.

Table IV:Phaseratio(β)*valuesforcommoncolumndimensions.Tomaintainsimilarseparations,choosecolumnswithsimilarphaseratioswhenchangingtoacolumnwithadifferentinnerdiameterorfilmthickness.

Film Thickness (df)Column ID 0.10 µm 0.25 µm 0.50 µm 1.0 µm 1.5 µm 3.0 µm 5.0 µm0.18 mm 450 180 90 45 30 15 90.25 mm 625 250 125 63 42 21 130.32 mm 800 320 160 80 53 27 160.53 mm 1,325 530 265 128 88 43 27


100% Dimethyl

Rxi®-1msRxi®-1HT

Rtx®-1

PolyethyleneGlycol


90% + Bis-Cyano

Rtx®-2330




6% Cyano Rtx®-624

Rtx®-1301



Rtx®-5



Proprietary

Rxi®-XLB

Proprietary

Rtx®-440

Tri�uoropropyl


14% Cyano

Rtx®-1701

50% Cyano

Rtx®-225

65% Diphenyl

Rtx®-65

20% Diphenyl

Rtx®-20

50% Phenyl

Rtx®-50





















Polar



*Phase ratio (β) = radius/2df (Note: Convert variables to the same units prior to calculation.)

Figure 4:Characteristicsandrecommendedapplicationsbasedoncolumninnerdiameter.

INNER DIAMETERID

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Inner Diameter (ID)

Compared to larger ID columns, smaller ID columns generate more plates per meter and sharper peaks, leading to better separation efficiencies. When more complex samples need to be analyzed, smaller ID columns can produce better separation of closely eluting peaks than larger ID columns. However, sample loading capacities are lower for smaller ID columns. Smaller ID columns, especially those at 0.18 mm and less, demand highly efficient injection techniques so that the column efficiency is not lost at the point of sample introduction. Column characteristics based on ID are presented in Table V.

Generally speaking, a 0.25 mm column will produce the most efficient sample analysis while simultane-ously considering analysis time and sample loading capacity. For these reasons, in combination with its relatively low outlet flow, it is also the best column choice for GC-MS work.

Consider Efficiency when Choosing Column Length, Column ID, and Carrier GasColumn Length

Capillary GC columns are made in various lengths, typically 10, 15, 30, 60, and 105 meters, depending on the inner diameter. Longer columns provide more resolving power than shorter columns of the same inner diameter, but they also increase analysis time and should be used only for applications demanding the utmost in separation power. Column length should only be considered once the stationary phase has been determined. This is because separation factor has the greatest effect on resolution, and it is maximized through proper stationary phase choice for the compounds of interest. Doubling the col-umn length (e.g., 30 m to 60 m) increases resolution by approximately 40%, while analysis time can be twice as long. In addition, longer columns cost more. Conversely, if a separation can be performed on a shorter column (e.g., 15 m versus 30 m), then both analysis time and column cost will be less. Figure 5 summarizes the characteristics and general application parameters for a range of typical column lengths.


100% Dimethyl

Rxi®-1msRxi®-1HT

Rtx®-1

PolyethyleneGlycol


90% + Bis-Cyano

Rtx®-2330




6% Cyano Rtx®-624

Rtx®-1301



Rtx®-5



Proprietary

Rxi®-XLB

Proprietary

Rtx®-440

Tri�uoropropyl


14% Cyano

Rtx®-1701

50% Cyano

Rtx®-225

65% Diphenyl

Rtx®-65

20% Diphenyl

Rtx®-20

50% Phenyl

Rtx®-50





















Polar



=R N14

kk+1

1X X

=R N14

kk+1

α-1X X

=R N14

kk+1

α-1X X

Figure 5: Characteristicsandrecommendedapplicationsbasedoncolumnlength.

LENGTH

ID

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Table V:GeneralcolumncharacteristicsbasedonID.Column Inner Diameter (mm)

Characteristic 0.10 0.15 0.18 0.25 0.32 0.53Nitrogen flow (mL/min) 0.2 0.3 0.3 0.4 0.6 0.9Helium flow (mL/min) 0.6 0.8 1.0 1.4 1.8 3.0Hydrogen flow (mL/min) 0.7 1.1 1.3 1.8 2.3 3.7Sample loading capacity (ng) 2.5 10 20 50 125 500Theoretical plates/meter 11,000 7,000 6,000 4,000 3,000 2,000

Carrier Gas Type and Linear Velocity

Carrier gas choice and linear velocity significantly affect column separation efficiency, which is best illus-trated using van Deemter plots (Figure 6). The optimum linear velocity for each gas is at the lowest point on the curve, where plate height (H) is minimized, and efficiency is maximized. As seen in Figure 6, the optimum linear velocities differ among common carrier gases.

Nitrogen provides the best efficiency; however, the steepness of its van Deemter plot on each side of opti-mum means that small changes in linear velocity can result in large negative changes in efficiency. Com-pared to nitrogen, helium has a wider range for optimal linear velocity, but offers slightly less efficiency. In addition, because of its optimum velocity being faster, analysis times with helium are about half those when using nitrogen, and there is only a small sacrifice in efficiency when velocity changes slightly. Of the three common carrier gases, hydrogen has the flattest van Deemter curve, which results in the short-est analysis times and the widest range of average linear velocity over which high efficiency is obtained.

Regardless of the type of gas used, the carrier gas head pressure is constant during column temperature programming, whereas the average linear velocity decreases during the run. For constant pressure work then, the optimal linear velocity should be set for the most critical separations. More commonly today, electronic pneumatic control (EPC) of carrier gas allows for constant flow or even constant linear veloc-ity, which helps maintain high efficiency throughout a temperature programmed run.

Another consideration for carrier gas type that is important, even if not directly related to column ef-ficiency, is whether a mass spectrometer (MS) is used as a vacuum-outlet detector for GC. In almost all cases, helium is the carrier gas of choice, not only for its chromatographic efficiency, but also because it is easier to pump than hydrogen. Hydrogen can be reactive in MS sources, leading to undesirable spectrum changes for some compounds. Nitrogen is typically not a carrier gas option for GC-MS, as it severely reduces sensitivity.

Figure 6: Operatingcarriergasattheoptimumlinearvelocitywillmaximizeefficiencyatagiventemperature.Redcirclesindicateoptimumlinearvelocitiesforeachcarriergas.

H

Average Linear Velocity

H2

He

N2

Van Deemter Plot

Note: Flows listed are for maximum efficiency. Sample loading capacities are estimates only. Actual sample loading capacity varies with film thickness and analyte.

Whenchangingcarriergasflowratesyoumustreconfirmpeakidentificationsaselutionorderchangescanoccur.

tech tip

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• Power supply• Electrical connections• Signal connections

• Gas purity• Gas flows• Temperature settings

• Syringe condition• Sample preparation• Analytical conditions

Check the Obvious:

• Define the problem clearly; for example, “Over the last 4 days, only the phenols in my sample have been tailing.”

• Review sample and maintenance records to identify trends in the data or problem indicators, such as area counts decreasing over time or injector maintenance not being performed as scheduled.

• Use a logical sequence of steps to isolate possible causes.

• Document all troubleshooting steps and results; this may help you identify and solve the next problem faster.

• Always inject a test mix and compare to previous data to ensure restored performance.

Identify the Cause:

Document Work and Verify System Performance:

Basic StepsFollow these basic troubleshooting steps to isolate problems related to the sample, injector, detector, and column. Check the obvious explanations first and change only one thing at a time until you identify and resolve the problem.

Example Troubleshooting SequenceAn analyst observed that no peaks appeared during a GC-FID analysis. The flowchart below shows a logical progression of steps that can be used to identify the cause and correct the problem.

No peaks from FID

Flame is lit.

There is �ow and the column

is not broken.

Re-inject Are there peaks?

ENDSyringe works and contains

sample.

NO

YES

NO

YES

NO

YES YES

Return detector to operating condition

and light �ame. Verify condensation is gone,

gases are on, and column is correctly

installed.

Adjust �ow or replace column.

Repair or replace syringe; ensure syringe is �lling.

Is the �ame lit?

Test/Check:• Water condensation?• Detector gases on?• Column installed correctly?• Plugged FID jet?

Is there adequate column �ow?

Test/Check:• Column �ow?• Broken column?

Is the sample reaching the column?

Test/Check:• Syringe plugged or broken?• Sample in syringe?

GC TROUBLESHOOTING TIPS

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Symptoms and SolutionsGood chromatography is critical to obtaining accurate, reproducible results. Coelutions, asymmetric peaks, baseline noise, and other issues are common challenges in the GC laboratory. These analytical problems and others can be overcome by troubleshoot-ing your separations using the tips below.

Poor Retention Time Reproducibility

• Leak check injector and press- t connections. • Replace critical seals (i.e., septa, O-rings, inlet disc, etc.)

• Maintain inlet liner and GC column. • Use properly deactivated liners, seals, and columns.

• Avoid sample overload. • Verify column temperature and oven temperature program.

• Verify the carrier gas ow and linear velocity. Repair or replace parts if neccessary.

• Con rm GC oven program falls within instrument manufacturer’s recommendation. • Extend GC oven equilibration time.

• Use autosampler or standardize manual injection procedure.

Causes

Leaks

Analyte adsorption

Resolution/integration issues

Incorrect column/oven temperature program

Incorrect or variable carrier gas ow rate/linear velocity

Poor control of oventemperature programming

Incorrect oven equilibration time

If manual injection, delay between pushing start and actual injection

Fronting Peaks

Poor Resolution

• Choose appropriate stationary phase and column dimensions. • Optimize carrier gas linear velocity and GC oven temperature program.

• Adjust sample concentration or amount on column. • Verify temperature program, ow rates, and column parameters.

Causes Solutions

Solutions

Solutions

Non-selective stationary phase

Poor e�ciency

Sample overload

Incorrect analytical conditions used

• Choose appropriate stationary phase. • Reduce amount injected, dilute sample. • Increase column inner diameter and/or �lm thickness.

Causes

Incompatible stationary phase

Column overloading

Tailing Peaks

• Use properly cleaned and deactivated liner, seal, and column. • Trim inlet end of column. • Replace column if damaged.

• Derivatize compound.

• Check for leaks at all connections, replace critical seals if needed.

• Minimize dead volume. • Verify that the column is cut properly (square). • Verify correct installation distances.

Adsorption due to surfaceactivity or contamination

Adsorption due to chemical composition of compound

Installation issues

Leak in system

Causes

Unstable Baseline (Spiking, Noise, Drift)

Spiking

Noise

Drift

• Leak check connections and replace seals if needed. • Replace carrier gas and/or detector gas �lters.

• Clean system and perform regular maintenance.

• Condition, trim, and rinse column.

• Replace septum. • Inspect inlet liner for septa particles and replace liner if needed.

• Clean and repair electrical connections.

• Verify �ow rates are steady and reproducible; may need to replace or repair �ow controller. • Leak check system. • Allow enough time for detector temperatures and �ows to equilibrate.

Carrier gas leak or contamination

Injector or detector contamination

Septum coring/bleed

Loose cable or circuit board connections

Variable carrier gas ordetector gas ows

Detector not ready

Column contaminationor stationary phase bleed

Causes

Carryover/Ghost Peaks

Injection 1

Injection 2

• Replace rinse solvent. • Rinse or replace syringe.

• Inject a smaller amount. • Use a liner with a large internal diameter. • Increase head pressure (i.e., owrate) to contain the vapor cloud. • Use slower injection rate. • Lower inlet temperature. • Increase split ow. • Use liner with packing. • Use pressure-pulse injection.

• Extend analysis time to allow all components and/or matrix interferences to elute.

Contaminated syringe or rinse solvent

Backash (sample volume exceeds liner volume)

Last analysis ended too soon

Causes

No Peaks

• Plugged syringe; clean or replace syringe. • Verify there is sample in the syringe. • Injecting into wrong inlet; reset autosampler. • Verify carrier gas is owing.

• Replace column.

• Re-install column.

• Signal not recorded; check detector cables and verify that detector is turned on. • Detector gas turned o� or wrong ow rates used; turn detector on and/or adjust ow rates.

Injection problems

Broken column

Column installed into wrong injector or detector

Detector problems

Causes

Split Peaks

• Adjust solvent or stationary phase to allow wetting.

• Add surface area, such as wool, to the inlet liner to enhance vaporization. • Use proper injector temperature.

• Inject less sample (dilute, use split injection, reduce injection volume).

• Use wool or slow injection speed.

Mismatched solvent/stationary phase polarity

Incomplete vaporization

Sample loading capacity exceeded

Fast autosampler injection into open liner

Causes

Broad Peaks

• Minimize dead volume in the GC system; verify proper column installation, proper connectors, proper liners, etc.

• Verify injector and detector ow rates and adjust if needed. • Verify make-up gas ow and adjust if needed.

• Increase GC oven programming rate.

• Lower GC oven start temperature. • Reduce retention of compounds by decreasing �lm thickness and length.

• See Carryover/Ghost Peaks solutions.

High dead volume

Low ow rates

Slow GC oven program

Sample carryover

Column �lm is too thick

Poor analyte/solvent focusing

Causes

High Bleed

• Increase conditioning time and/or temperature.

• Trim column and/or heat to maximum temperature to remove contaminants. • Replace carrier gas and/or detector gas �lters. • Clean injector and detector.

• Check for oxygen leaks across the entire system and replace seals and/or �lters. • Replace column.

Improper column conditioning

Contamination

Leak in system and oxidation of stationary phase

Causes

Response Variation

• Check sample concentration. • Check sample preparation procedure. • Check sample decomposition/shelf life.

• Replace syringe. • Check autosampler operation.

• Verify signal settings and adjust if needed. • Repair or replace cables or boards.

• Perform detector maintenance or replace parts.

• Verify steady ow rates and temperatures, then adjust settings and/or replace parts if needed.

• Remove contamination and use properly deactivated liner, seal, and column.

• Check for leaks at all connections and repair connections as needed.

• Verify injection technique and change back to original technique. • Check that split ratio is correct. • Verify that the splitless hold time is correct.

Sample issues

Syringe problems

Electronics

Dirty or damaged detector

Flow/temperature settings wrongor variable

Adsorption/reactivity

Leaks

Change in sample introduction/injectionmethod

Causes

Solutions

Solutions

Solutions

Solutions

Solutions

Solutions

Solutions

Solutions

12 www.restek.com 1-800-356-1688or1-814-353-1300








Causes

Leaks

Analyte adsorption







Fronting Peaks

Poor Resolution



Causes Solutions

Solutions

Solutions


Poor e�ciency

Sample overload



Causes


Column overloading

Tailing Peaks







Installation issues

Leak in system

Causes


Spiking

Noise

Drift









Septum coring/bleed



Detector not ready


Causes


Injection 1

Injection 2







Causes

No Peaks


• Replace column.



Injection problems

Broken column


Detector problems

Causes

Split Peaks









Causes

Broad Peaks






High dead volume

Low ow rates


Sample carryover



Causes

High Bleed





Contamination


Causes

Response Variation









Sample issues

Syringe problems

Electronics




Leaks


Causes

Solutions

Solutions

Solutions

Solutions

Solutions

Solutions

Solutions

Solutions

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Causes

Leaks

Analyte adsorption







Fronting Peaks

Poor Resolution



Causes Solutions

Solutions

Solutions


Poor e�ciency

Sample overload



Causes


Column overloading

Tailing Peaks







Installation issues

Leak in system

Causes


Spiking

Noise

Drift









Septum coring/bleed



Detector not ready


Causes


Injection 1

Injection 2







Causes

No Peaks


• Replace column.



Injection problems

Broken column


Detector problems

Causes

Split Peaks









Causes

Broad Peaks






High dead volume

Low ow rates


Sample carryover



Causes

High Bleed





Contamination


Causes

Response Variation









Sample issues

Syringe problems

Electronics




Leaks


Causes

Solutions

Solutions

Solutions

Solutions

Solutions

Solutions

Solutions

Solutions

CheckouttheRestekblogforthemostcurrenttopicsinchromotography.

blog.restek.com

14 www.restek.com 1-800-356-1688or1-814-353-1300








Causes

Leaks

Analyte adsorption







Fronting Peaks

Poor Resolution



Causes Solutions

Solutions

Solutions


Poor e�ciency

Sample overload



Causes


Column overloading

Tailing Peaks







Installation issues

Leak in system

Causes


Spiking

Noise

Drift









Septum coring/bleed



Detector not ready


Causes


Injection 1

Injection 2







Causes

No Peaks


• Replace column.



Injection problems

Broken column


Detector problems

Causes

Split Peaks









Causes

Broad Peaks






High dead volume

Low ow rates


Sample carryover



Causes

High Bleed





Contamination


Causes

Response Variation









Sample issues

Syringe problems

Electronics




Leaks


Causes

Solutions

Solutions

Solutions

Solutions

Solutions

Solutions

Solutions

Solutions

1-800-356-1688or1-814-353-1300 www.restek.com 15








Causes

Leaks

Analyte adsorption







Fronting Peaks

Poor Resolution



Causes Solutions

Solutions

Solutions


Poor e�ciency

Sample overload



Causes


Column overloading

Tailing Peaks







Installation issues

Leak in system

Causes


Spiking

Noise

Drift









Septum coring/bleed



Detector not ready


Causes


Injection 1

Injection 2







Causes

No Peaks


• Replace column.



Injection problems

Broken column


Detector problems

Causes

Split Peaks









Causes

Broad Peaks






High dead volume

Low ow rates


Sample carryover



Causes

High Bleed





Contamination


Causes

Response Variation









Sample issues

Syringe problems

Electronics




Leaks


Causes

Solutions

Solutions

Solutions

Solutions

Solutions

Solutions

Solutions

Solutions

GC Troubleshooting Tips Simplifying Column Selection

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